Aight, then you must have met Martin a few times since the battle at kilo point.

So the last time you saw him must have been at least a few years ago. Back then he tried to explain to Jack the following:

Gabriel is his father, he gave him the potential and power. The point where developed his current vampiric self is when he almost died and spend time in the Terebithia.
So it makes sense in a Martin thinking sort of way to draw the connection between the Terebithia's laboratory tubes and a mother's womb. So While Gabriel is his father, the 'logical' conclusion is that the Terebithia is his mother. And that would make Jack his grandfather.

I'm not sure where I'm going with this, but I'm sure Martin has thought of this and if he did he would have shared that with Jack. Because of reasons.